Ultra high frequency inductor



June 11, 1946. w. J. POLYDOROFF ULTRA HIGH FREQUENCY INDUCTOR Filed Jan. 27, 1945 dare-Malawi;-

INVENTOR W fr auzv Patented June 11, 1946 2,401,882 uuras men msounncr mpuc'roa Wladimir J. Polydorofl, Chicago, Ill., ass'ignor to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application January 21, 1943, Serial No. 414,046 In Canada May 22, 1942 (Cl. I'll- 242) 13 Claims. 1

The present invention relates to high frequency circuits or tunable or adjustable type in which the inductance elements of the circuits are ad- :Iustable by means of ferro-magnetic cores.

Circuits including inductors oi the above-mentioned type are, in general, applicable incases where ultra high frequencies are to be dealt with, and particularly so in high frequency radio or the like transmitting equipment where the losses introduced through the use of iron cores must be kept as low as possible.

It has been a common practice in the inductance coils for such circuits to space the turns apart and to employ core materials of low loss nature. Both of these expedients greatly reduce 2 sistivity above 10,000 ohm-ems. does not produce appreciable improvement. The insulating films between the particles are so minute that there always is danger of breaking down the insulation under strong currents and by the heat developed in the core material during operation at high frequencies. To prevent such breakdowns in cores operating in high tension coils, additional insulatthe amount of inductance variation obtainable 1 by the movement oi the core in and out 01! the coil, so that the tuning range is greatly restricted. I

The present invention has for one of its objects an improved construction of coil and core in.

which large inductance variations are obtainable while low core losses are maintained.

Another object of the invention is to improve tuning curve or variation 01' inductance, vs. linear displacement of the core.

Still another object of the invention is to provide such convenient designs which permit desired characteristics at high currents.

The invention h generally applicable to high frequency transmitting and receiving circuits when the usually employed coils are heiically spacedly wound and the core material of low permeability is used and its utility extends to a trequency range from Ito 100mm The inv'entionwili be better understood by re!- ercnce to the accompanying drawing in which Fig. 1 shows a conventional type of variable ferrO-inductor to which. the invention is appli-- cable: 1

Fig. 2 shows diagrammatically the inductance variation obtainable in the device shown in Fig. 1:

Fig. 3 shows an improved variable ierro-inductor according to the invention;

Fig. 4 shows diagrammatically the inductance variation obtainable in the device shown in Fig. 3;

'Fig. 5 shows one application or an improved inductor according to the invention to an ultra high frequency circuit, and

Fig. 6 shows the invention inits application to transmitting equipment.

In the production of cores of common practice, composite core resistivity oi. a very high order is usually maintained to restrict the circulation of currents from one particle to another, practice ing means are provided.

In the present invention, in order to .iurther improve the operation 01' cores at higher frequencies or under strong currents, the cores may be made of powdered magnetic materials of low conductivity, a particularly suitable magnetic material of this kind being synthetically prepared pure magnetite (FeaOobut it possesses low permeability. Alloying of iron with silicon or antimony in high proportion of the non-magnetic agent also materially decreases the conductivity of the particles and thus reduces the losses.

The use of a low permeability core material necessitates different constructions of the inductors to compensate for the reduction of the effective permeability obtainable by such a core in a high frequency coil. Further, it has been deflniteiy established that in high irequeency coils usually'employing spaced turns the eii'ective permeability is still more reduced because oi incomplete linkage from turn to turn due to the leak- .age held, this leakage becoming more pronounced Depending upon the nature oi its employment, the

coil maybe irom 2" to it" in diameter and consist of several spaced turns of solid or hollow wire.

The core structure may be in the form 0! a cylinder' or a cone. As already stated. the spacing between the turns reduces the eiiective permeability oi the coil, yet this spacing is necessary to provide an elongated coil in which the eflective permeability is usuallyincreased, also to facilitate smooth adjustment or the inductance with linear movement or the core. Were it possible to showing, however, that an increase of such rell place the turns closer t ether while maintainins acumen the same length-to-cliameter ratio, a higher effective permeability would be obtained.

For this reason alone we should use inductance as high as possible. However, the circuit considerations limit the value of the inductance to a value consistent with the amount of capacitance, residual or external, necessary to resonate the circuit at a given frequency. Thus we arrive at inductance values which become exceedingly small at frequencies approaching 100 megacycles. In practice such inductance can be realized with a single turn of I. D. which in turn will require an adjusting cylindrical core of approximately O. D. While this is applicable to a receiving circuit, the conditions met with in a transmitting circuit are of a more serious nature because, from the consideration of heat dissipation in the coil and the core, the diameters should be considerably increased and even one turn oi wire will be too much to form a circuit of satisfactory L/C ratio for the above frequencies.

The inductance variations due to the coremay be considered as consisting of two parts: one, the

- inductance variation in the turn or turns per se,

and the other, the variation of mutual inductance or linkage between the turns, both variations adding together, the latter variation decreasing as the turns are spaced apart.

Let us consider now the inductance variation due to the change of inductance oi the turn. Fig. 2 shows schematically one turn of inductance I and its variation due to the core 2. Curve a of S-type shows a rather abrupt), change 01 inductance when one face of the core passes through a turn and as the core progresses through the turn the inductance change becomes negligible.

Fig. 3 shows an adjustable ferro-inductor in accordance with the invention in which the winding is arranged in a novel manner, by taking advantage of well-known inductances in parallel. Two coils 1, of the same diameter and inductance, are shown side by side and connected together so as to form a parallel combination in aiding relation. Neglecting for the moment the mutual inductance between the two coils, the inductance of each parallel section, 1. e. each coil, should be doubled in order to obtain the same inductance as in the case 01 Fig. 1.-

Assuming that inductance is proportional to the square of the number of turns it will be seen that each section requires an increased number oi turns by the factor of 1.41 so that the whole coil combination has approximately three times as many turns as before. This also means that for the same geometrical dimensions the turns of each section must be spaced from one another by one-third of the spacing required in the previous example, which, as already explained, considerably improves the linkage between the turns at high frequency and consequent effective permeability. The choice of proper diameter of the wire (hollow tubing) for a satisfactory coil for high frequency calls for the use of relativeiy thick wire which means that the effective coil diameter with respect to the core is increased. This also causes decrease of effective permeability. When two coils in parallel are provided, as described it is permissible to decrease size of wire, which becomes another contributing factor to the increased inductance variation. It is or course understood that the above reasoning equally well applies to paralleling of a several scctioz'ls together so that a still greater number of turns may he used. in selecting the method laying parallel sections together one must bear in mind (a) that all sections must be laid in aiding relation, lb) that the sections with their connecting wires have substantially equal inductance, and (c) that the sections are laid symmetrically. Fig. 3 shows such a symmetrical arrangement for two sections and Fig. 5 shows the same method applied to a ii-section coil.

Referring now to Fig. 4 and neglecting again inductance variation due to change of mutual or linkage inductance between the turns, we may consider the inductance variation produced by the same core in an inductor having two turns in parallel. The curve b shows the variation of inductance from minimum to maximum for the same length of travel of the core as in Fig. 2. The curve now becomes more gradual, thus enabling improved selection of frequencies if the core is used as a tuning element. While the total inductance variation of the inductor due to change of inductance in the turns per se remains substantially the same as in Fig. 2, the additive mutual inductance variation separately shown by curve 0 accounts for an improved total inductance variation d. The same line of reasoning applies to a coil having a plurality of turns wound in several parallel sections, as shown in Fig. 5. This figure also shows the application of a multi-section coil to an U. H. F. oscillator B of the Colpitts type.

In the design of transmitter coils, enough surface must be provided to secure dissipation oi heat generated in the coil at high frequencies. The above described expedient usually adds to the metal surface of the coil from which heat is more readily radiated and results in cooler operation of the system.

A core composed either of metallic particles insulated from each other, or of a non-metallic magnetic substance, is usually a poor conductor of heat and in spite of all precautions may develop considerable heat inside of its mass when used in a transmitting circuit. These design considerations are shown in Fig. 6. In this figure a transmitter coil 3 having a ribbon type of winding is shown. This provides a coil having a large surface, the winding consisting of two parallelconnected sections and the core 2 .being formed of a plurality of sections 4 with air spacing between them and mounted upon an insulating rod, 5. The comminuted particles may be bound together by an inorganic binder, for instance, of ceramic nature, to avoid deterioration of core at elevated temperatures.

It has been observed that the coil alone has a certain drift of inductance at high temperatures, which drift is somewhat increased when the core is also heated. In term-inductors having low permeability material, such as above described. the temperature coefficient may be estimated as being approximately percent for F. rise. As transmitter coils may easily reach this temperature increase during operation, it is recommended, in circuits of the kind described, that fixed capacitors having an equal and opposite temperature coefficient be employed in order that the circuits may remain stable at different temperatures of operation.

The invention may be employed to an advantage in the receiving circuits calling for considerable range of frequencies, such as for instance 6-18 mo. band. In such circuit the minimum into employ a cylindrical core closely fitting in the inside of the tube and of a length slightly in excess of 1".

In that frequency range the core material must be of relatively low permeability in order to reduce the core losses, hence the employment of special windings is recommended. In a construction as described the total inductance variation of 3:1 may be realized so that the whole range is covered in two steps: 6 to 10 mo. and 10 to 18 mc., the second range being obtainable by disconnecting part of a capacitor or by switching the sections of the windings or parts of same from a series to a parallel combination.

If further increase in inductance range is desired within the same geometric limitations of the coil, more sections can be wound, each to still higher .inductance and parallel together as per Fig. 5, so that still thinner wire and more turns in the coil will produce higher inductance variation.

As another extreme application of the present invention to ultra high frequency circuits may be the case. Normally, a single turn inductance will be employed. In order to further reduce the inductance value and therefore increase the operating frequency of the inductor it is proposed in accordance with this invention to use a wide metal ribbon formed as a single turn, the width of the ribbon being slightly less than'the length of the adjusting core. Additionally, this single turn may be slotted transversely to its axis so that in efl'ect several turns-are spaced from each other by the width of the slots, and joined at the bottom of each turn by common metal thus forming a. parallel connection as-already explained in Fig. 4. The practice shows that theinductance and the losses in such c011 are both greatly reduced and the effective permeability due to the core is noticeably increased. The slots cut in the solid metal may be so varied in their shape and length that all sectional turns are of substantially the same inductance.

Having thus described my invention, what I claim is:

1. A variable ferromagnetic inductor including a coil and a moveable ferromagneticcore of low permeability, said coil and core being co-axial and said coil comprising a plurality of identical solenoidal sections, each of said sections being wound in opposite direction to the next adiacent section, said sections being positioned end to end along the common axis and connected in parallel in aiding sense.

2. A ferromagnetic inductor as claimed in claim 1 wherein the core is composed of particles of h h resistivity.

3. A variable ferromagnetic inductor'including a coil and moveable ferromagnetic'core, said coil and core being'coaxial and said coil comprising a plurality of solenoidal sections. said sections being positioned end to end along the common axis and connected in parallel in aiding sense, said core being composed or discsections separated from each other toimprov'e the heat sipation of said core.

4. A variable ferromagnetic inductor suitable disthrough paralleling of several identical sections of much higher inductance, said sections being positioned end to end along the common axis of said coil and said core to thereby greatly increase the number of turns in said inductance traversed by said core during its movement, said identical sections being wound in opposite direction tothe next adjacent section.

5. An inductance device comprising a plurality of coils disposed end to end and electrically connected in parallel, and a magnetic core arranged for axial movement with respect to said coils.

6. An inductance device comprising a plurality of coils disposed end to end and electrically connected in parallel with their magnetic flux mutually aiding, and a comminuted magnetic core arranged for axial movement with respect to said coils.

'7. An inductance device comprising a plurality of coils wound on a form and disposed end to end, said coils being electrically connected in parallel with their magnetic flux mutually aiding, and a comminuted magnetic core adapted to be moved axially within thecoil form.

8. An inductance device comprising a plurality of short coils disposed end to end and electrically connected in parallel, a coil form on which the several coils, are wound, and a magnetic core adapted to be moved axially within the coil form.

9. An inductance device comprising a plurality of short coils disposed end to end and electrically connected in parallel with their magnetic flux mutually aiding, a 4 coil form on which the several coils are'wo'und, and a comminuted magnetic core adapted to be moved-axially within the coil form.

10. An inductance device comprising a coil having a plurality of windings which are wound together on a form, the turns of said windings being spaced and only the terminals of the windings being electrically connected to effect a parallel connection of the windings with their magnetic flux mutually aiding, and a comminuted magnetic core arranged for movement within the coil form.

11. An adjustable magnetically tuned resonant circuit, comprising a plurality of coils electrically connected in parallel and with their m netic flux mutually aiding, and an adjustable magnetic core cooperating with said coils, the arrangement being such that the operating range of said circuit is extended beyond that obtainable with a single coil having substantially the'same inductance as the plurality of parallel coils.

12. An adjnstable magnetically tuned resonant circuit, comprising a plurality of coils co-axially disposed end to end, electrically connected in parallel and withtheir magnetic flux mutually aidins, and an adjustable magnetic core cooperating with said coils, the arrangement being such that the operating range of said circuit is extended beyond that obtainable with a single coil having substantially the same inductance as the plurality of parallel coils.

13. An adjustable magnetically tuned resonant circuit, comprising a coil having a plurality of windings only the terminals of which are elecfor ultra high frequencies including a coil and a trically connected to connect the several windings in parallel with their magnetic flux mutually aiding, and an adjustable magnetic core cooperating with said windings. the arrangement being such that the operating range or said circuit is extended beyond that obtainable with a single coil winding having substantially the same inductance 0| the plurality of parallelcoil windings.

IMIR J. POLYDOROFF. 

